I-Team lead investigative reporter Jayne Miller said Friday that sources told her the explosion, which sealed off a four-block radius of the downtown area for a while, causing traffic gridlock, was caused by an M-80 firecracker.

The Baltimore City Sheriff's Office, which is in charge of the investigation, said the incident started with the sound of an explosion at the entrance of the courthouse at 111 N. Calvert St. around 2:30 p.m. Thursday.

Authorities, including city police and fire, the Baltimore City Sheriff's office, the Baltimore City Office of Emergency Management, the Bureau of Alcohol, Tobacco, Firearms and Explosives, the Federal Protective Service police, hazmat crews, a bomb robot and K-9 units rushed to the scene, where they found a trash can outside the courthouse that had exploded.

In a statement released Friday, the Baltimore City Sheriff's Office said, "In a collaborative effort the area was systematically cleared and declared safe."

Emergency crews quickly sealed off four blocks around the building and used a robot to check for other possible devices in the area, but nothing was found. The sealed-off area was reopened a short time later. Downtown traffic was gridlocked for a short time but eventually started to flow again.

"The Sheriff’s Office preliminary examination of the evidence at the scene indicates that there was a low impact type of item which is characteristic of a firework. While this item created a loud noise and white smoke, all damage was limited to the inside of the trashcan. There were no injuries or damage to any buildings or items in the surrounding area," the statement said.

Baltimore City Police Commissioner Anthony Batts on Friday said the incident should be taken seriously, especially in light of the bombs that went off at the Boston marathon earlier this year.

"You have to be diligent, much like we were in 2001 and 2002. We have to go back to that and even a trash can around a court building, we need to check those," Batt said.

Batts said it's a great reminder to tighten up public places.

"We have to make sure we have K-9 doing the perimeter or the sheriffs have to do that because that's their responsibility. I think the message is with that firecracker or not, we have to be diligent," Batts said.

Trash cans have been removed from many public spaces as a means of deterring the placement of explosive devices. They remained in place Friday outside the courthouses.

A man who was working on some scaffolding at a nearby monument at the time of the explosion explained to 11 News what he witnessed.

"Well, we were standing up top on the top rung of the scaffolding. We just heard a loud bang, about as loud as a shotgun, and looked down and there was a small square trash can, about up to my knees, and the lid had been blown off and there were some scraps. I don't know if it was aluminum, but some shards of metal and it was smoking. There were a few people around. It wasn't anything too crazy," said Justin, who did not give a last name. "Nobody was hurt."

He added, "It exploded. Something blew up inside of it, definitely, because there was definitely smoke billowing out of the top of it. People were just covering their ears."

The Baltimore City Sheriff's Office said it is still trying to figure out what the intent of the M-80 was. The office said it plans to review surveillance video Friday to try to get clues as to who may have put the firecracker there.

While hundreds of people were inside the courthouse at the time, investigators said they did not evacuate the building initially because they were told the device was contained to the trash can and there was no real threat to the public; however, officials did say they eventually evacuated as a precaution.

The Baltimore City Sheriff's Office in the statement said the ATF is investigating what type of device was used. They also said a criminal investigation is underway and investigators are trying to identify a suspect.

"This crime is being taken very seriously and your best course of action is to immediately turn yourselves in to law enforcement authorities," Sheriff John W. Anderson said.

Tipsters can provide anonymous information on the above crime by calling the Metro Crime Stoppers hotline available 24 hours a day toll free at 866-7LOCKUP to be eligible for up to a $2,000 reward for information leading to an arrest and conviction.

Stay with WBALTV.com and watch 11 News for more on this developing story.

OUT AND IT WAS DETONATED IN DEEMED NOT HAZARDOUS. NO INJURIES OR EVACUATIONS WERE REPORTED. WE HAVE LEARNED THAT IT WAS A VERY BIG, VERY POWERFUL FIRECRACKER THAT CAUSE THE EXPLOSION IN A TRASH CAN OUTSIDE OF THE MITCHELL COURTHOUSE. IT SHUT DOWN THE AREA AROUND THE COMPLEX YESTERDAY AFTERNOON AND WE HAVE THE LATEST ON THE INVESTIGATION FROM JAYNE MILLER JOINS US LIVE DOWNTOWN. FORTUNATELY THIS DID NOT HAVE THE FORCE OF AN EFFECT OF WHAT HAPPENED EARLIER THIS YEAR IN BOSTON. YET A CRIMINAL INVESTIGATION IS UNDERWAY. THE POLICE COMMISSIONER SAYS THIS IS A GREAT REMINDER OF WHAT NEEDS TO BE DONE TO TIGHTEN UP SECURITY IN PUBLIC PLACES. IT WAS BUSINESS AS USUAL AT THE CIRCUIT COURT COMPLEX DOWNTOWN. NO TRACE OF THE INCIDENT THAT STOPPED EVERYTHING YESTERDAY AFTERNOON. IT WAS IN MAD THAT WENT OFF AT THE ENTRANCE TO THE MITCHELL COURTHOUSE. -- IT WAS AN M-80. IT IS A LARGE AND POWERFUL FIRECRACKER. VIDEOS POSTED TO YOUTUBE SHOW WHAT THEY CAN DO WHEN THEY GO OFF. THE INCIDENT YESTERDAY SHOULD BE TAKEN SERIOUSLY ESPECIALLY IN LIGHT OF THE BOMBS THAT WENT OFF AT THE BOSTON MARATHON EARLIER THIS YEAR. YOU NEED TO BE DILIGENT. WE HAVE TO GO BACK TO THAT. IT EVEN A TRASHCAN AROUND A COURT BUILDING, WE NEED TO MAKE SURE THAT WE HAVE CANINES DOING PARAMETERS OR THE SHERIFFS BECAUSE THAT IS THEIR RESPONSIBILITY BUT THE MESSAGE IS THAT WE HAVE TO CONTINUE TO BE DILIGENT. TRASH CANS HAVE BEEN REMOVED AS A MEANS OF DETERRING THE PLACEMENT OF EXPLOSIVE DEVICES. THEY REMAIN IN PLACE TODAY OUTSIDE THE COURTHOUSE. THE CITY SHERIFFS DEPARTMENT IS THE LEAD AGENCY INVESTIGATING THIS INCIDENT AND POLICE ARE ALSO INVOLVED AS WELL. ATF AND THE CITY SHERIFF ASKED THE PERSON RESPONSIBLE TO TURN